High-temperature stainless steel



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continuation-in-part of my copending application, Serial No. 671,905 filed May 23, 1946, and entitled-High Temperature Stainless Steel, now Patent No. 2,447,897 of August24,' 1948, which application in turn is a continuation-in-part of my copending application, Serial No. 645,015 flledFebruary 1, 1946, and entitled High Temperature Turbine, now Patent No. 2,447,896, and the invention relates to high temperature steel andproducts and articles 01 the steel. 1 e

An object of my invention is the provision of strong and durable stainless steel and products and articles of the steel having highly satisfactory resistance to stress-rupture and creep while hotand under load, and which a.e well suited for resisting attack by hot corrosive matter and the formation of heat scale.

Another object is the provision of high temperature stainless steel of the character indicated which has good hot-working properties.

A further object of my invention is the provision of a simple, practical and reliable method for conditioning stainless steels to a point of enhanced resistance to high temperature stressrupture and creep. c

A still further object is that of providing stressrupture resistantand creep resistant high temperature stainless steel subjected to heat treatment for enhancing the high temperature proporties.

Other objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the comblnation of elements, composition or materials, and in the articles, product and manufactures, also in the several method steps and the relation of, each of the same to one ormore of the others as described herein, the scope of the application of which isindicated in the following claims.

tain featuresof my invention, it may be noted at this point that austenitic chromium-nickel stainless steels of the ordinary grades falling within the range of to chromium, 7% to 15% nickel contents; and the remainder substantially alliron,have heretofore been widely in the form of, low temperature or mildly heat resistant products or articles of manufacime; By virtueoi the presence of nickel, these stainless steels have a relatively high alloy con tent as compared with straight chromium ferritic 6 Claims. (01. IS-425i i There is still the existing fact, however, that austeniticstainless steels or the characterjust noted are'unsatisfactory for meeting exacting de mands at high temperatures whereused ror ex-- ample as bolts and'fasteners, internal combustion engine valves, gas and steam 1 turbine blades, rotors, buckets, nozzles, supercharger components; or as any of a host olj other products and articles subjected to load while extremely hot. Such products, when made oi these steels are too susceptible to creep and stress-rupture under the severe heat.

The conventional austenitic chromium-nickel 7 stainless steels referred to, accordingly, have gained popular demand in thetorm of such low temperature products as trim for house, store. omce, or restaurant purposes, or vehicle trim and other products capable of resisting the effects of weather. These steels too are very much in demand as cooking utensils, table ware, receptacles and other appliances where a metal of enduring so As conducive to a clearer understanding of oerbrightness i needed to withstandbody salts, mm; acids and other corrosive compounds, An ong the better known products of this class are those made of the usual 18% chromium- 8% nickel steels which are readily worked as by rolling, f

drawing, spinning, and the like, to desirediorm.

While the austenitic stainless steels or the character noted give betterall-around service at low temperatures, they still have a more favorable lattice structure for cohesion under stress or load at elevated temperatures than do i'erritic straight-chromium stainless steels. This comparison, however, does not necessarily assure adequate high-temperature properties, especially with respect to such considerations as resistance to stress-rupture and creep.

The austenitic chromium-nickel steels in most "instances do have favorable hot working properties, which so often is not true of certain heretoi'ore known high temperature creep resistant and 1 connection, many high temperature steels, for

3 stainless steelarticles ahdlproducts which resistantto heatand corrosion atelevated temfl peratures and are resistant to creep and stress example, are notoriously difllcult to hot work.

An outstanding object-of my invention, accordinely, is the provision offhot-worked "austeniticf o rupture under mechanical'load and sever hea'ti j stainless steels and are bettersuited tores'st R e r now m r P r i arl to h Pr -y; tice of my invention" I find"that by ccrre etmg; proper amounts of the ingredientschroniifuml I nickel, cobalt, manganese. molybdenum, copper, F. carbon, titanium and columbium to provide an 3 austenitic stainless steel, satisfactory hot-work ing properties are had along with excellent high temperature strength and resistance to heat and corrosion. The steel which I provide, importantly contains about 12% to 22% chromium. 0.5% to nickel, from 4% to 40% cobalt, manganese from 0.1% or less up to about 2%. 1% to 4% molybdenum, from 2% to 4% copper, carbon preferably in the range of 0.01% to 0.35%,

from 0.15% to 0.75% titanium, 0.20% to 1.10%."

columbium, and the remainder substantially all iron. 'Such elements as sulphur and phosphorus preferably are less than about 0.04% each.

I often form my steel by such steps as hot working and machining, into any of a host of high temperature products and articles, among which are'bolts, fasteners, rivets, chemical equipment parts, tubes, such as seamless tubes formed by piercing and drawing. gas and steam turbine blades, rotors, buckets, nozzles and supercharge parts for serving under mechanical stress or resisting corrosion during their intended use. The

high temperature products and articles of manufacture'a're strong, durable and reliable. They resist creep and stress-rupture while hot.

There are occasions where I provide products of the steel in such form as sheet, strip, wire, rods, or the like, which are readily useful for fabrication as by cutting, punching. bending, or welding as by means of oxy-acetylene or arc welding equipment, into desired shape. Sometimes, too, I use the sheet, wire, or the like, directly in high temperature applications without appreciable further fabrication. 1

The steel and articles and products made from the same are wholly austenitic. Ferrite, if present at all, is only in traces. This I find is essential to the required stress-rupture properties. Where appreciable amounts of ferrite are present the stress-rupture values fall 01!; also the working properties suffer. The substantial quantity of cobalt which I employ not only supplements the effect of nickel to assure an austenitic balance, but improves the stress-rupture prop erties and serves to cut down development of sigma phase. Also, with the cobalt constituent, a steel of relatively high alloy content is had in favor of better scaling resistance under intense heat.

Withany appreciable lowering of the copper and molybdenum-contents to outside the ranges hereinbefore noted, the high temperature load In heat treating forenhancing the high temperature properties, I include operations in the form of annealing and precipitation treatment. To effect this combined treatment, I heat up the steel, as for example, roughly shaped articles thereof, first to temperatures where titanium and columbium go into solution. These temperatures preferably range from about 2050 F. to 2250' F.- The titanium is quite soluble throughout this heating range'and the columbium becomes more soluble toward the upper end of the range. Copper also goes into solution.

Working operations are achieved on the metal. if desired, either before. along with, or following the annealing treatment.

Upon treating the steel at solution temperature for a sufficient period of time, I thereafter quench the steel as in air, oil or water, conveniently to about room temperature. Subsequently, I heat the quenched steel up to a temperature preferably within the range of about 1200 F. to 1500 P. where a critically dispersed, finely divided pre-' cipitate comes out in the metal lattice along the slip planes in the matrix. In this, there is a precipitation of intermetallic compounds including titanium and columbium. Copper may come out in fine form, or it may appear as an intermetallic compound comprising such elements as nickel, titanium and columbium. Some portion of the precipitated elements, such as the titanium and columbium is in the form of carbides which increase in amount toward the high side of-the precipitation temperature range just noted.

At the comp.etion of the prec.pitation heat treatment, I quench the steel. The quenched metal has a fine grain structure, and is further.

characterized by enhanced load-carrying capacity in view of atomic slip-interference deve.oped

by the precipitates. The steel in this conditionoften is worked, fabricated or flushed to give articles or products. During high temperature use of the steel, the precipitates remain critically Table I. Chromium-nickel-cobalt stainless steels Steel CMn P's SiICr NiMo Cu 00 TiCo carrying capacity of my steels suffer, and with All of the steels in Table I were subjected to a appreciable increase beyond the ranges, worksolution heat-treatment at 2250 F. for one-half ability disappears. The elements titanium and columbium in the amounts indicated enhance the stress-rupture and creep properties, and give improvements in the high temperature load-carrying ability, particularly after-proper heat treatment of the steel.

hour followed by quenching in water plus precipitation heat-treatment at 1350" F. for five hours and quenching in air. After the anneal-- ing and precipitation treatments, samples of these steels were given stress-rupture tests with the results noted in T0010 11.

Load (p. s. i.)

Load (p. s. i.)

Em iurcd at Engiured at Steel 1200 E, for 1500 F. for

Hours Hours 17, 000 11,000 18,000 13, 700 18,000 11.500 18, 000 I4, 000 17,000 14, 000 20. 000 15,000 17, 200 13, 200 20, 500 14, 600 19,800 14, 200 18, 000 12,000

My austenitic chromium-nickel-cobalt stainless steels have many valuable properties, including resistance to stress-rupture and resistance to creep whether or not the steels and products and articles thereof are in the precipitation heattreated condition. The precipitation heat treatment enhances a number of the properties by the development of atomic slip-interference. Also, my steels, despite the high temperature properties of the same are capable of fabrication with relative ease as compared with many high temperature steels of the ferritic grade.

Thus it will be seen that there are provided in this invention austenitic chromium-nickelcobalt alloy stainless steel-and products thereof in which the various objects noted together with many thoroughly satisfactory results are successfully achieved. It will be seen that the products are tough, strong and durable, corrosion-resistant and heat-resistant and serve well at high temperatures over long periods of time under many conditions of actual practical use.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth, it will be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. Austenitic chromium-nickel-cobalt stainless steel. containing about 12% to 22% chromium, at least but less than nickel, from 4% to 40% cobalt, from small quantities up to about 2% manganese, 1% to 4% molybdenum, 2% to 4% copper, 0.01% to 0.35% carbon, from 0.15% to 0.75% titanium. 0.20% to 1.10% columbium, and the remainder substantially all iron.

2. Hot workable high temperature stainless steel, comprising 12% to 22% chromium, at least 5% but less than 10% nickel, from 4% to 40% cobalt, from small quantities up to about 2% manganese, 1% to 4% molybdenum, 2% to 4% copper, 0.01% to 0.35% carbon, from 0.15% to 0.75% titanium, 0.20% to 1.10% columbian, sulphur and phosphorus each not exceeding about 0.04%, and the remainder substantially all iron, said steel being substantially fully austenitlc and 6 susceptible to precipitation and critical dispersion of a finely divided precipitate including titanium, columbium and copper from solution to enhance high temperature load carrying capacity.

3. Austenitic chromium-nickel-cobalt stainless steel, containing about 12% to 22% chromium. at least 5% but less than 10% nickel, from 4% to 40% cobalt, 1% to 4% molybdenum, 2% to 4% copper, carbon ranging from 0.01% to 0.35%. from 0.15% to 0.75% titanium, 0.20% to 1.10% columbium, and the remainder substantially all iron.

4. Wrought austenitic high temperature stainless steel articles having great strength while hot, said articles comprising about 12% to 22% chromium, at least 5% but less than 10% nickel, cobalt in amounts ranging from 4% to 40%, from small amounts up to 2% manganese, from 2% to 4 copper, 1% to 4% molybdenum, carbon ranging from 0.01% to 0.35%, titanium 0.15% to 0.75%, from 0.20% to 1.10% columbium, and the remainder substantially all iron.

' 5. Austenitic high temperature stainless steel containing about 12% to 22% chromium, at least 5% but less than 10% nickel, cobalt in amounts ranging from 4% to 40%, from small amounts up to 2% manganese, 1% to 4% molybdenum, from 2% to 4% copper, carbon in the range of 0.01% to 0.35%, from 0.15% to 0.75% titanium, 0.20% to 1.10% columbium, and the remainder substantially all iron, said copper, titanium and columbium being at least partially precipitated in critically dispersed form for slip-interference.

6. Wrought austenitic high temperature stainless steel articles having great strength and resistance to creep while hot, said articles comprising about 12% to 22% chromium, at least 5% but less than 10% nickel, cobalt in amounts ranging from 4% to 40%, from small amounts up to 2% manganese, 1% to 4% molybdenum, from 2% to 4% copper, 0.01% to 0.35% carbon, titanium rang ing from 0.15% to 0.75%, columblum from 0.20% to 1.10%, and the remainder substantially all iron, said copper, titanium and columbium being at least partially precipitated in critically dispersed form for slip-interference.

WILLIAM CHARLES CLARKE, Js.

Resonances crrsn The following references are of record in the file oi. this patent:

UNITED STATES PATENTS Number Name Date 2,397,034 Mohling Mar. 19, 1948 2,447,897 Clarke Aug. 24, 1948 OTHER REFERENCES Progress Report on Heat Resisting Metals for Gas Turbine Parts. P. B. 39579. Published September 21, 1943, by War "Metallurgy Division of the National Defense Research Committee of the Oilice of Scientific Research and Development. Declassifled February 18, 1948. 

1. AUSTENITIC CHROMIUM-NICKEL-COBALT STAINLESS STEEL, CONTAINING ABOUT 12% TO 22% CHROMIUM, AT LEAST 5% BUT LESS THAN 10% NICKEL, FROM 4% TO 40% COBALT, FROM SMALL QUANTITITE UP TO ABOUT 2% MANGANESE, 1% TO 4% MOLYBDENUM, 2% TO 4% COPPER, 0.01% TO 0.35% CARBON, FROM 0.15% TO 0.75% TITANIUM, 0.20% TO 1.10% COLUMBIUM, AND THE REMAINDER SUBSTANTIALLY ALL IRON. 